Usb: f_gsi: Add uevent support for connect/disconnect events

Currently gsi driver indicates the state change by by sending zero length
packet. Send uevents to indicate to specific gsi protocol connect and
disconnect state as well.

Change-Id: Icfad5ccba74b7f33b1bacad685d4eb15520e72b5
Signed-off-by: Vamsi Krishna Samavedam <vskrishn@codeaurora.org>